1. Field of the Invention
The present invention generally relates to an inductive radio communication system, and particularly to an inductive radio communication system for transmitting and receiving a signal, such as audio data, video data, or the like, between a mobile body or unit and a ground station.
2. Description of the Related Art
Such an inductive radio communication system of the type described above has heretofore been known to have the following configuration. As shown in FIG. 1 of the accompanying drawings, parallel lines 2 are laid down so as to serve as an inductive radio cable along a track of a mobile unit 1. An antenna 3 attached on the mobile unit 1 is arranged in the vicinity of the parallel lines 2. A signal to be transmitted from the mobile unit 1 to a ground station is modulated by a transmitter 4, then the modulated signal is supplied to the antenna 3 and transmitted from the antenna 3 to the parallel lines 2 due to electromagnetic coupling therebetween. The signal transmitted to the parallel lines 2 is then supplied through a transformer 5 to a receiver 6 of the ground station side and then demodulated in the receiver 6. Thus, the demodulated signal is output as a reception signal from the receiver 6. FIG. 2 shows an outer face of an example of the parallel lines 2.
In such an inductive radio communication system as described above, the antenna 3 on the mobile unit 1 is moved in a manner that it opposes to the parallel lines 2 on the ground. When the mobile unit 1 is moved, since the antenna 3 is displaced in the upper and lower direction due to vibrations of the mobile unit 1, the distance between the antenna 3 and the parallel lines 2 is changed. There is then the problem that, when the antenna 3 is moved in the direction that the distance becomes large, the coupling level of the electromagnetic coupling is lowered.
In order to solve the above-mentioned problem, there is proposed a method that the output power of the transmitter side is increased to a certain extent taking the fluctuation of the above-mentioned distance into consideration, or an antenna having a good radiation efficiency is used. Such a method, however, is not preferable because undesired radiation noise radiated to the outside increases accordingly.